Tools for medium- and long-term prediction of nourishments effects

In this study the effect of different nourishment designs at the medium- (years, kilometres) and large- (decades to centuries, tens to hundreds of kilometres) scale was investigated by means of numerical tools and data analysis. The effects were evaluated by looking at a number of morphological indicators of simple use for coastal managers: the MKL position, the dune foot position and the beach width. Two types of models were used as a basis: the standard cross-shore UNIBEST-TC model, with the addition of the beach-dune module to simulate changes in the dry part of the profile, and the Nourishment Impact tool (alongshore coastline model). The UNIBEST-TC model was used to simulate the effects of a large set of different nourishments scenarios (-300) and the results were then analysed by means of a Bayesian approach. Observations on the efficiency of different nourishment designs were derived for the different indicators at different time scales. The comparison with data showed similar trends than the ones derived using the models. Although still qualitative, the results in terms of bar migration prediction in combination with nourishments have also appeared to be promising. The coastline model Nourishment Impact Tool was used to predict the coastline development of the Dutch coast for 30 years, using as input the data of the nourishments which were carried out during this period. The model was able to predict reasonably well the large scale MKL development. Nevertheless, the prediction of the dune foot position at the large scale could not be represented. Also the prediction of the morphological changes at the local scale could not be represented by the tool, which contains a large number of simplifications and was originally developed only to simulate the effects of the long-term and large-scale nourishment programmes.